This invention relates to an elastic wave generator, magnetostriction oscillator mounting structure and method.
An elastic wave generator is a device for generating an acoustic longitudinal elastic wave and expected to be utilized in the following variety of fields:
(1) a transmitter of an acoustic transmission apparatus utilizing a drilling pipe as an acoustic signal transmitting medium;
(2) a transmitter of an acoustic transmission apparatus utilizing a metal structural body or a rigid structural body as an acoustic signal transmitting medium;
(3) an apparatus for injecting or internally generating a longitudinal elastic wave into a concrete or metallic structural body for inspecting characteristics, property changes, flaws or the like of the structural body;
(4) an apparatus for injecting or internally generating a longitudinal elastic wave into a concrete or metallic structural body for structural analysis of a building or a mechanical structural body;
(5) a seismic wave generating apparatus for evaluating stratum or the like;
(6) an acoustic speed measuring apparatus for measuring characteristics of strata or the like; and
(7) other apparatus for non-destruction inspection, measurement, survey, information transmission or the like in which a relatively large vibration is effective.
Heretofore, the method for applying a stress to a mechanical structure includes a hydraulic press generating a static compression stress, an expansion test machine generating a static tensile force or a hammer or a falling object generating a dynamic stress. In the method using the hammer or the falling object, it is difficult to control the stress at will so that the detailed examination of the workpiece cannot be carried out.
On the other hand, a method for converting strain into stress as a method for freely controlling the stress is used in acoustic measurement of a relatively small structure having a relatively small thickness or uniform composition. However, since this method is not suitable for use in terms of a large structure or a sparse material which needs large stress, the measurement of these materials had to be achieved by generating a large stress with a hammer or a falling substance.
Therefore, in order to generate a controlled large stress, an attention has been paid to materials that have extremely high strain characteristics and a strain-stress conversion apparatus utilizing these materials (piezoelectric material, supermagnetostriction material) has been studied. However, these materials, such as brittle and weak sintered alloys or easily bendable and deformable materials, have compression strengths that are too small not to self-destruct upon the generation of the strain in the stressed state.
While it has been known that the generated stress can be made large when the strain is generated by energizing the piezoelectric material or the magnetostriction material put under the pre-stressed state, the compression strength of the material was too small and the material was self-destroyed when a strain is generated even with a small pre-stress. Therefore, it was not possible to obtain at will a large stress, such as a stress of 10-30 kgf/mm2, in order to generate a strong elastic wave (acoustic signal), so that this material was not suitable to apply to a large structural body or a sparse material.
Therefore, while a strain-stress conversion device employing these materials (piezoelectric material, super magnetostriction material) having high strain characteristics have been used in the acoustic measurement of a relatively thin matter or a structural body of a homogeneous composition, the measurement with the hammer or the falling object has conventionally been used for a large structural body or the materials of sparse composition. Although the realization of the acoustic transmission through the use of the drilling pipes for oil rigs of a length of from several hundred meters to several thousand meters have been believed, since there has been provided no realistic acoustic longitudinal elastic wave generator that can provide a large stress wave, the pressure wave transmission or mud pulse system in which drilling mud is used as the transmission medium, has been employed.
According to the study and the analysis of the inventors of the present invention, the unsolved technical problems in these conventional fields of application are given below:
(1) generation and injection of a stress wave at will in a large metallic structural body;
(2) injection of a stress wave at will into a structural body having a composition of an internal disturbance reflection such as concrete, rock or plastic; and
(3) generation and injection of an acoustic longitudinal wave of from several Hz to several ten kHz at and into a structural body.
Then, the present invention is expected to be utilized in a wide variety of fields as discussed previously, so that both the present invention and the conventional technique will be described in terms of examples in which they are applied to the field of oil rigs.
For example, Japanese Patent Laid-Open No. 8-130511 discloses the system in which the information of the bottom of the well being dug in an oil well is transmitted by an elastic wave (acoustic) signal using the drill pipes as a transmission medium.
FIGS. 23 to 25b are views showing the system disclosed in Japanese Patent Laid-Open No. 8-130511 and FIG. 23 being a view showing the overall construction of the oil well facility, FIG. 24 being a fragmental sectional view showing the drill collar portion at the well bottom of the drill pipe, FIG. 25a being a plan view and FIG. 25b being a sectional side view.
In FIGS. 23-25b, 100 shows a tower for drilling an oil well. 24 is a drill pipe inserted into the ground from the tower 100 and having straight cylindrical pipes each having a length of about 8 meters, connected to constitute an assembly of a length of several hundred meters to several thousand meters, the drill pipe being driven to rotate by an unillustrated drive unit mounted to the well tower 100. Disposed within the drill collar 22 of the drill pipe 24 at the bottom portion of the well is a magnetostriction oscillator 34. 25 is a sensor for detecting various information necessary for digging, the various information signals from the sensor 25 at the well bottom are converted into digital signals so that an electric current varying in accordance with the converted digital information signal is supplied to an excitation coil 36 of the magnetostriction oscillator 34, which converts the signal into an elastic wave (acoustic signal) to be transmitted through the drill pipe 24 to the ground surface. 26 is a receiver installed in the portion of the tower 100 of the drill pipe 24, 27 is a signal processing apparatus for processing to demodulate the received signal of the receiver 26 to monitor the state of the bottom of the well, such as the temperature of the well, the tilt of the drill tip or the like.
The mounting of the magnetostriction oscillator 34 to the drill collar 22 is achieved by the cantilevered mounting as shown in the sectional side view of FIGS. 25a and 25b in which a clamping screws 39 and rock nuts 40 are used to make a canti-lever attachment (canti-levered system), so that when the magnetostriction oscillator 34 is elongated by energization, the reaction force from the inertia weight 42 disposed at the free end side is transmitted to the drill collar 22 through the horn 38 to become an elastic wave. Therefore, the injection efficiency of the elastic wave (acoustic) energy from the magnetostriction oscillator 34 to the drill pipe on the order of 0.01 to 0.1, so that, when the drill pipe 24 of several hundred meters to several thousand meters long is used as a transmission medium in actuality, the elastic wave (acoustic wave) does not reach the receiver 26 at the ground level and this system is not practically used in oil well drilling, but the pressure wave transmission or mud pulse system, in which drilling mud is used as a transmission medium, is utilized.
Thus, according to the elastic wave generating device employing the conventional magnetostriction oscillator 34, it is difficult to generate and inject a large stress wave (elastic wave) of a desired size in a large structural body.
The present invention has been made in order to solve the above-discussed problems and has as its object the provision of an elastic wave generator, a magnetostriction oscillator mounting structure and a mounting method free from the above-discussed problems.
With the above objects in view, the present invention resides in an elastic wave generator comprising: an excitation coil; a magnetostriction oscillator around which the excitation coil is wound and made of a lamination of magnetostriction sheets having a metallic crystalline structure which exhibits positive strain characteristics in which its length varies directionally upon magnetic excitation; and an oscillator support having a first support surface shrink-fit against a first end surface of the magnetostriction oscillator intersecting the direction along which the length of the magnetostriction oscillator changes and a second support surface shrink-fit against a second end surface of the magnetostriction oscillator intersecting the direction along which the length of the magnetostriction oscillator changes, whereby the changes in the length of the magnetostriction oscillator due to the magnetic excitation of the excitation coil appearing at the first and second end surfaces is directly supported by the first and second support surfaces. Therefore, the elastic wave generator can be realized in which the generation and the injection of a large stress wave (elastic wave) that has not heretofore been solved.
The elastic wave generator may comprise an excitation coil; a magnetostriction oscillator around which the excitation coil is wound and made of a lamination of magnetostriction sheets having a metallic crystalline structure which exhibits positive strain characteristics in which its length varies directionally upon magnetic excitation; a magnetic bias device having a magnetic path in common with the magnetostriction oscillator; and an oscillator support having a first support surface shrink-fit against a first end surface of the magnetostriction oscillator intersecting the direction along which the length of the magnetostriction oscillator changes and a second support surface shrink-fit against a second end surface of the magnetostriction oscillator intersecting the direction along which the length of the magnetostriction oscillator changes, whereby the changes in the length of the magnetostriction oscillator due to the magnetic excitation of the excitation coil appearing at the first and second end surfaces is directly supported by the first and second support surfaces. Therefore, the elastic wave generator can be realized in which the generation and the injection of a large stress wave (elastic wave) that has not heretofore been solved and the operating point can be arbitrarily set.
The present invention also resides in an elastic wave generator mounting structure for mounting a magnetostriction oscillator to an object to which an elastic wave is irradiated, the magnetostriction oscillator comprising an excitation coil wound around a stack of thin sheets of a metallic magnetostriction material bonded together with an electrically insulating bonding agent for generating an elastic wave in the direction parallel to the thin sheet by an excitation current flowing through the excitation coil; the magnetostriction oscillator having two parallel surfaces intersecting at right angles with an elastic wave radiation direction and spaced apart from each other by a distance A at room temperature and a distance A1 at a lower temperature; the object having a hole or a recess having two parallel wall surfaces intersecting at right angles with the elastic wave radiation direction and spaced apart from each other by a distance B at room temperature; a relationship among the distances being A greater than B greater than A1; and the magnetostriction oscillator being held in the hole or recess by the shrink-fit against the wall surfaces in which the magnetostriction oscillator is cooled and contracted and then returning to room temperature to expand the magnetostriction oscillator within the hole or recess. Therefore, the magnetostriction oscillator is held in the pre-stressed state by the shrink-fit, so that the pre-stress incomparably stronger than that obtained by the conventional tightening screw can be applied.
The present invention also resides in a method for mounting a magnetostriction oscillator to an object to which an elastic wave is irradiated, the magnetostriction oscillator comprising an excitation coil wound around a stack of thin sheets of a metallic magnetostriction material bonded together with an electrically insulating bonding agent for generating an elastic wave in the direction parallel to the thin sheet by an excitation current flowing through the excitation coil; the method comprising: a magnetostriction oscillator shaping step for shaping two opposing elastic wave radiation surfaces formed by stacking the thin sheets into two parallel surfaces intersecting at right angles with an elastic wave radiation direction and spaced apart from each other by a distance A at room temperature; an object shaping step for providing a hole or a recess having two parallel wall surfaces intersecting at right angles with the elastic wave radiation direction and spaced apart from each other by a distance B at room temperature between two wall surfaces at room temperature which is smaller than the distance A; a cooling step for cooling the magnetostriction oscillator until the distance A becomes equal to a distance A1 smaller than the distance B of the hole or the recess; and an insertion step for inserting the cooled magnetostriction oscillator into the hole or recess. Therefore, the magnetostriction oscillator is held between the wall surfaces of the recess or the hole in the shrink-fit state due to the expansion at the room temperature before the cooling, so that it is held in the pre-stressed state due to the shrink-fit that is incomparably stronger than that obtained by the conventional tightening screw can be applied.